other aerobatic displays which produced a corps of skilled pilots who would contribute to military air forces during World War II on all sides of the conflict. Recreational gliding flourished, particularly in Germany through Rhön-Rossitten. In the US, the Schweizer brothers manufactured sport sailplanes to meet the new demand. Sailplanes continued to evolve through the 1930s and sport gliding became the main application of gliders. Military aviation In military aviation, the fast all-metal monoplane emerged slowly. During the 1920s the high-wing parasol monoplane vied with the traditional biplane. It was not until the arrival of the American Boeing P-26 Peashooter in 1932 — nearly fifteen years after the first low-wing fighter to enter limited military service, the all-metal airframe Junkers D.I had entered service with the Luftstreitkräfte in 1918 — that the low-wing monoplane began to gain favour, reaching its classic form in such designs. These were pioneered in late 1933 by the Soviet Union with the Polikarpov I-16 fighter, powered initially with an American Wright Cyclone nine-cylinder radial engine. Within only a few years after the I-16's first flights, the German Messerschmitt Bf 109 of 1935 and the British Supermarine Spitfire of 1936 were also flying, powered by new and powerful liquid-cooled vee-twelve engines respectively from Daimler-Benz and Rolls-Royce. The rotary engines common in the First World War quickly fell out of favour, being replaced by more powerful stationary air-cooled radial engines such as the Pratt and Whitney Wasp series. References Inline citations General references Almond, P.; Aviation: The early years, Ullmann, 2013 edition. Between Interwar period